1. Field of the Invention
This invention relates to an f.theta. lens for use in the scanning optical system of a facsimile, a printer or the like.
2. Description of the Prior Art
In the scanning optical system of a facsimile or the like, for example, a laser beam modulated in accordance with image signals is deflected in a predetermined plane by using, for example, a polygonal reflecting member, and the reflected light beam from this reflecting member is imaged in a predetermined image plane by an imaging lens. Control of the deflection velocity of the light beam by this polygonal mirror, if effected with the angular velocity being constant, is simpler in mechanism and much lower in cost. Thus, the imaging lens is required to have a performance which provides an image height H in the imaging plane directly proportional to the deflection angle .theta. of the light beam.
Generally, in a convergent type lens system, the angle of incidence .theta. on the lens system and the image height H in the image plane of the lens system are related such that H=f tan .theta.. Where .theta. is in a small range, .theta. can be regarded as tan .theta..apprxeq..theta., but in the angle of view required of the scanning optical system, the range of tan .theta.&gt;0 predominater and in order that H=f.theta. may be exactly provided, the imaging lens must be endowed with negative distortion.
There are two types of f.theta. lens system, namely, the telecentric type in which the an entrance pupil and the forward focus of the f.theta. lens system are made coincident with each other, whereby the principal ray of the light beam after having passed through the f.theta. lens system becomes parallel to the optical axis of the f.theta. lens system, and the type in which the entrance pupil is set inwardly of the forward focus of the f.theta. lens system so that the principal ray of the light beam after having passed through the f.theta. lens system spreads with respect to the optical axis of the f.theta. lens system.
The f.theta. lens system of the telecentric type is excellent in performance for linearity of scanning because the principal ray of the imaging light beam becomes perpendicular to the image plane and thus there is no deviation of the imaging height even for any deviation of the image plane, while on the other hand it has the disadvantages that if the scanning width becomes great, the f.theta. lens becomes bulky in proportion thereto and that the full length from the entrance pupil to the image plane becomes great. The f.theta. lens system of the latter type has a disadvantage in that the linearity of scanning becomes poor for any deviation of the image plane because the imaging light beam is inclined with respect to the image plane, while on the other hand it has the advantages that even if the scanning width is made great, the f.theta. lens does not becomes so bulky and the full length from the entrance pupil to the image plane can be made relatively short. The present invention is concerned with this latter type of f.theta. lens system.
The performance of an f.theta. lens can be considered in terms of the angle of view as relating to the scanning width, the imaged spot size as relating to the imaging performance, and the linearity of scanning. With regard to the f.theta. lens system used in a scanning optical system, it is possible to reduce its cost and enhance its productivity by allowing the occurrence of spherical aberration and coma up to the amount of diffraction determined by F-number in view of the fact that the incident beam diameter is small as compared with the total focal length of the f.theta. lens system, that is, the effective F-number is great, by allowing the linearity of scanning to vary up to several percent and by using, for example, an f.theta. lens system of two-lens construction. However, if the incident beam diameter becomes greater and the effective F-number of the f.theta. lens system becomes smaller, it is difficult to correct the various aberrations of the f.theta. lens system by a two-lens construction and, thus, a lens system of three-lens or more-lens construction is usually used. Even in such case, if the linearity of scanning is sacrificed and allowed to vary up to about 1%, a two-lens construction will be permissible up to the order of an F-number of 70. On the other hand, if the F-number is greater than 70 and the variation in linearity of scanning becomes as severe as about 0.5%, a two-lens construction will be impossible and a lens system of at least a three-lens construction will be required. Depending on the scanning optical system, there are many f.theta. lens systems in which the scanning width is long and the angle of view is as wide as about 45.degree. and linearity of scanning of as high accuracy as 0.05%-0.01% is required.